1. Field of the Invention
The invention relates to a fixing apparatus applicable to electrophotographic image forming apparatuses, such as copiers, printers and facsimile machines, and machines that combine their functions.
2. Description of the Related Art
Various fixing apparatuses have been known in the prior art as one exemplified by the disclosure of JP-A-6-19345, comprising: a thin heat resistant film; and a heating device that is located and held fixedly on one side of the film while grasping the film; and a pressure roller that is located on the other side of the film opposing the heating device and pressing via the film against the heating device an image carrying side of recording paper where images are to be fixed. In said apparatus, the pressure roller rotates to feed the film and the recording paper at the same speed and fixation is done through heating.
Another type of fixing apparatus proposed by JP-A-6-75493 is equipped with: a heat roller containing a heater internally; a thin sleeve that is located on the outside of the heat roller and has an larger inner diameter than the outer diameter of the heat roller; and a pressure roller that is located to contact under pressure with the heat roller via the thin sleeve.
In these fixing apparatuses, the recording paper, on which the unfixed toner image is transferred to, passes through a nip area formed between the thin heat resistant film or the thin sleeve and the pressure roller, and the toner is fixed on the recording paper by means of pressure and heat applied during the passage. This means that the nip width can be widened and a lower heat capacity can be used. Consequently, it is possible to make the fixing apparatus smaller and reduce the power consumption.
However, in the fixing apparatus disclosed by JP-A-6-19345, the film is rotated by the drive force of the pressure roller while the film's inner face is contacting the heating device, so that there is a possibility that the driving force of the pressure roller is not effectively transmitted to the film when the recording paper is being fed, thus affecting the uniform rotation of the film.
As a means of solving this problem, several ideas have been proposed such as: providing a gear-like elastic protrusion on the end of the pressure roller; and increasing the friction coefficient of a portion of the film surface. However, even with these means, there is a possibility of irregular film rotation if the drive force is not properly transmitted from the pressure roller to the film.
Moreover, in the fixing apparatus disclosed by JP-A-6-75493, it is necessary to transmit the drive force of the heat roller-to the pressure roller through the rotation of the thin sleeve in order to rotate the pressure roller. Therefore, the drive force transmitted from the heat roller to the pressure roller through the thin sleeve is not appropriate, slippage may occur on the pressure roller thus deteriorating the rotation.